I'd like to give you a more solid answer, but a lot of this will depend on where the word is in a sentence or where you are in Japan.

With this particular example, I can tell you that people in the Kansai area pronounce it SuKOSHi in most instances. But it could be different elsewhere.

In general, though, I wouldn't fret about it too much. You'll pick this sort of thing up as you converse with Japanese speakers. Until then, pronouncing all of the vowels is perfectly okay as a "safe side" sort of thing when you just don't know.